1. Field in the Industry
The present invention concerns high speed tool steel called “matrix type high speed steel”. The invention provides the steel of this kind, in which dispersion of the properties after heat treatment is small and high toughness can be always obtained.
2. Prior Art
It is often practiced that forging dies and press forming dies used in hot working and metal working dies and roll forming dies used in cold working are made with matrix type high speed steel which is a die material of high strength. Examples of the matrix type high speed steels are M50, M52 and so on, which are standardized in AISI. In Japan it has been proposed to use a high speed steel such as SKH51 as the basic alloy and lower the contents of carbon, molybdenum and tungsten therein so as to decrease the quantity of carbides formed therein and to improve the toughness (Japanese Patent Publications No. S50-10808 and Japanese Patent Disclosure No. S61-213349).
However, in case where the above-mentioned known materials are used for dies of cold metal working, it is often observed that very strong strain is locally posed on the dies and therefore, the molds may easily be broken in early stage of use without enjoying full lives. Even in dies for warm forging, working temperature is so controlled to obtain the products having better quality and thus, high load may be posed on the forging dies. Under these circumstances it is the current status that, using the conventional materials, the die lives may not be constant.
The inventor wished to break through the status of technology and sought a solution. He considered at the beginning of his development as follows. First, in order to prevent drastic destroy of the tools used in the state of high hardness and to ensure constant long lives, it is necessary to prevent formation of coarse carbides which may be the starting points of fracture, and therefore, the alloy design should be so made that the possibility of forming coarse carbides may be low. Then he noted that, in the present technology, without controlling the temperature range of quenching in such a narrow range as 10° C. or so, it will be difficult to ensure the hardness of the steel after heat treatment. Such controlling is not easy to realize in practical operation and thus, properties of the product tools have wide dispersion. Solution for this problem is, he considered, to make variation of solid-solution behavior of the carbides small even at various heat treatment temperatures. Further, because hardness and toughness greatly varies by the manner of cooling at quenching (or cooling rates), it is inevitable that the properties of the products vary depending on the sizes of the products. The inventor's conclusion in this regard was to aim at alloy compositions which give constant properties even if the cooling rates vary.
Based on the above-described analysis the inventor chose the following measures:
1) For the purpose of decreasing formation of coarse carbides, in view of the facts that the coarse carbides existing at the time of solidification are those of MC-carbides mainly of VC, it will be effective to decrease the content of vanadium and to hold the steel in sufficient soaking (for example, to keep at a temperature of 1200° C. or higher for 10 hours or more) so as to have the carbides dissolved.
2) In order to lower the sensitivity to heat treatment temperature it is effective to avoid drastic change of carbon solution at ordinary temperature range of quenching (1100° C. to 1200° C.) by making the structure at equilibrium condition γ+MC-phase or γ-single phase. For this, it is essential to appropriately control the Mo- and W-content as the balance of components.